Since legislation on exhaust-gas emissions is becoming increasingly stringent, the automotive industry is faced with high requirements, which are being met by appropriate measures aimed at reducing the raw emissions of engines and by an appropriate after-treatment of the exhaust gas. The introduction of European emission standard Euro 6 for gasoline engines stipulates a limit value for the particulate number which, in many cases, makes it necessary to employ a gasoline particulate filter. During driving, such a gasoline particulate filter is loaded with soot. This gasoline particulate filter has to be regenerated continuously or periodically so as to prevent the exhaust-gas counter-pressure from rising excessively. In order to use oxygen to carry out a thermal oxidation of the soot captured in the gasoline particulate filter, it is necessary to have a sufficiently high temperature in conjunction with the concurrent presence of oxygen in the exhaust system of the gasoline engine. Since modern gasoline engines are normally operated without an oxygen excess at a stoichiometric air-fuel ratio (λ=1), additional measures are necessary for this purpose. Possible measures include, for instance, raising the temperature by shifting the advance angle, temporarily shifting towards a lean mixture in the gasoline engine, blowing secondary air into the exhaust system or a combination of these measures. So far, preference is given to shifting the advance angle in the “late” direction in combination with shifting towards a lean mixture in the gasoline engine since this method makes do without the need for additional components and can deliver a sufficient amount of oxygen in most operating points of the gasoline engine. Efforts are also aimed at bringing the catalytic converters located in the exhaust passage of the internal combustion engine up to the operating temperature as quickly as possible after a cold start so that a high conversion rate for harmful exhaust-gas components can be reached as quickly as possible. However, a shift towards a lean mixture in the internal combustion engine during the regeneration of the particulate filter can cause a significant rise in the nitrogen oxide emissions (NOx emissions) during the regeneration of the particulate filter since, at an over-stoichiometric air-fuel ratio, the exhaust gas no longer contains components such as carbon monoxide (CO) or unburned hydrocarbons (HC) which allow a catalytic reduction of the nitrogen oxide emissions to form molecular nitrogen on the three-way catalytic converter.
German patent application DE 10 2013 220 899 A1 discloses a method for the regeneration of a particulate filter in the exhaust system of an internal combustion engine, whereby the temperature of the exhaust gas of the internal combustion engine is adapted by an appropriate lambda regulation in order to provide the temperature needed for the regeneration of the particulate filter and in order to ensure the concurrent presence of residual oxygen in the exhaust gas of the internal combustion engine. A drawback of such a method, however, is that a shift towards a lean mixture in the internal combustion engine leads to an increase in the nitrogen oxide emissions in the exhaust gas during the regeneration of the particulate filter.
International patent application WO 2015/169958 A1 discloses an externally ignited internal combustion engine whose exhaust system contains a particulate filter, whereby, in order to regenerate the particulate filter, a multi-stage process is carried so that any detrimental impact on the driving comfort during the regeneration of the particulate filter is kept to a minimum. Towards this end, a passive regeneration of the particulate filter during an overrunning phase of the internal combustion engine as the “mildest” measure is combined with appropriate active measures for the regeneration, whereby, in a multi-stage process, the specific measures selected are the ones involving the least impact on the driving comfort or on the performance of the internal combustion engine. A drawback of such a method, however, is that here as well, the active measures cause the air-fuel ratio of the internal combustion engine to shift towards a lean mixture, which is associated with an increase in the nitrogen oxide emissions.
German patent application DE 10 2012 022 153 A1 discloses a method for the regeneration of a particulate filter in the exhaust passage of a gasoline engine, whereby, in a first step, the temperature of the particulate filter or the temperature of an exhaust-gas mass flow through the particulate filter is determined and, when the particulate filter has reached a regeneration temperature, the soot captured in the particulate filter is oxidized in that the oxygen fraction in the exhaust gas is raised above the level of a stoichiometric exhaust gas.
German patent application DE 10 2010 046 899 A1 discloses the regeneration of a particulate filter in the exhaust passage of an internal combustion engine, whereby the regeneration takes place while the internal combustion engine is being switched off. In this process, the air-fuel ratio is shifted towards a “lean” mixture directly before the ignition is switched off when the particulate filter has reached a certain load and the temperature of the particulate filter is high enough to allow a partial regeneration of the particulate filter when the internal combustion engine is being switched off. The proposed method, however, only allows the regeneration of a small portion of the soot captured in the particulate filter since the time period during which oxygen-rich air is being conveyed into the exhaust passage after the injection and the ignition have been turned off is relatively short.
Moreover, exhaust after-treatment systems are known in which the temperature of the particulate filter is raised by shifting the advance angle in the direction of “late” until the particulate filter has reached its regeneration temperature and the amount of oxygen needed for the oxidation of the soot captured in the particulate filter is fed to the exhaust passage via a secondary air system. For this purpose, however, a secondary air system is always necessary, which markedly increases the costs for the internal combustion engine.